nir: split out instruction comparison functions

nir: split out instruction comparison functions

Right now nir_instrs_equal() is tied pretty tightly to CSE, but we're
going to introduce the idea of an instruction set and tie it to that
instead.  In anticipation of that, move this into its own file where
we'll add the rest of the instruction set implementation later.

v2: Rebase on texture support.

Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>
Signed-off-by: Connor Abbott <cwabbott0@gmail.com>